The present invention relates to communication systems. More particularly, it concerns optical communications systems in which a plurality of terminal devices communicate with one another over a common optical bus.
In many types of communications networks, it is common for the terminal devices to introduce information bearing signals onto a network bus by modulating a locally available carrier source. In the electrical domain, stable and highly accurate carrier sources are available that can be tuned from one frequency to another to allow frequency division multiplexed networks. In optical systems, however, the traditional carrier sources, viz., light emitting diodes and laser diodes, have inherent limitations which mitigate against high density frequency division multiplexed networks. As is known, laser diodes are difficult to tune from one wavelength to another and to wavelength stabilize, in part, because of their temperature sensitivity, and light emitting diodes are generally considered low power devices having a relatively wide spectral output. While frequency stabilization schemes are known, the many variables involved make the tuning of the local light source from one frequency to another difficult. While relatively stable light sources, such as gas lasers, are available, their comparatively high cost, physical size, power supply requirements, and the attendant difficulty of modulating their output makes them unattractive for use in each terminal device in the network.
As can be appreciated, any optical communications system which provides high density frequency division multiplexing without using a local carrier source in each terminal device possesses significant cost and performance advantages over prior systems.